How to create an instance variable with FactoryBot on Rspec

I want to test a class function on RSpec on my Product class. To ease the reading I will keep it as this:

class Product < ApplicationRecord

  private 

  def self.order_list(sort_by)
    if sort_by == 'featured' || sort_by.blank?
      self.order(sales: :desc)
    elsif sort_by == 'name A-Z' 
      self.order(name: :asc)
    elsif sort_by == 'name Z-A'
      self.order(name: :desc)
    elsif sort_by == 'rating 1-5'
      self.order(:rating)
    elsif sort_by == 'rating 5-1'
      self.order(rating: :desc)
      elsif sort_by == 'price low-high'
      self.order(:price)
    elsif sort_by == 'price high-low'
      self.order(price: :desc)
    elsif sort_by == 'date old-new'
      self.order(:updated_at)
    elsif sort_by == 'date new-old' 
      self.order(updated_at: :desc)
    end
  end
end

Once the function is called with a parameter, depending on the parameter that has been used, the list of products is ordered in a different way for the user to see.

I built a FactoryBot for the product model as well:

FactoryBot.define do
  factory :product do
    sequence(:name) { |n| "product_test#{n}" }
    description { "Lorem ipsum dolor sit amet" }
    price { rand(2000...10000) }
    association :user, factory: :non_admin_user
    rating { rand(1..5) }

    trait :correct_availability do
      availability { 1 }
    end

    trait :incorrect_availability do
      availability { 3 }
    end

    #Adds a test image for product after being created

    after(:create) do |product|
      product.photos.attach(
        io: File.open(Rails.root.join('test', 'fixtures', 'files', 'test.jpg')),
        filename: 'test.jpg',
        content_type: 'image/jpg'
      )
    end

    factory :correct_product, traits: [:correct_availability]
    factory :incorrect_product, traits: [:incorrect_availability]
  end
end

Basically, we want to call the :correct_product to create a product thats accepted by the validations of the model.

For the specs:

describe ".order_list" do
    let!(:first_product) { FactoryBot.create(:correct_product, name: "First Product" , rating: 1) }
    let!(:second_product) { FactoryBot.create(:correct_product, name: "Second Product" , rating: 2) }
    let!(:third_product) { FactoryBot.create(:correct_product, name: "Third Product" , rating: 3) }

    it "orders products according to param" do
      ordered_list = Product.order_list('rating 1-5')
      expect(ordered_list.all).to eq[third_product, second_product, first_product]
    end
  end

So basically, my question is, how can I create an instance variable for each of the 3 mock products so I can name them here in the order I expect them to appear: expect(ordered_list.all).to eq[third_product, second_product, first_product]

Or, even better, is there a way to create the instance variables with a loop and actually have the instance variable name to use them in the expect? This would free me from having to create 3 different variables on FactoryBot as I did.

I searched around the web and instance_variable_set is used in some cases: Testing Rails with request specs, instance variables and custom primary keys Simple instance_variable_set in RSpec does not work, but why not?

But it doesn´t seem to work for me. Any idea on how could I make it work? Thanks!